1. Technical Field
The present invention relates to a palladium catalyst with a new structure and its preparation as well as applications, and more especially to a multi-purpose, high-activity n-heterocyclic carbene (NHC) type palladium catalyst with a brand new structure, and its preparation as well as purposes in a plurality of coupling reactions.
2. Description of Related Art
Transition metal catalyzed C—C bond coupling reaction is a very effective organic synthesis means through which C—C bond can be formed at specific location under relatively mild conditions. Therefore, it can be applied in the synthesis of a variety of natural products, drug intermediates and organic materials, being of great significance to both academic research and industrialized development. Among these, transition metal palladium catalyzed cross-coupling reaction grows especially fast (Org. Process Res. Dev. 2005, 9, 253).
Organic metal palladium catalysts keep being developed over the years, leading to revolutionary changes of transition metal catalyzed C—C bond coupling reaction. Under the action of these new-type catalysts, aromatic halides, class of aromatic halides, alkenyl halides and the like can have good coupling reaction with various alkene, alkyne, aromatic compounds or organic metallic reagents. Wide functional group tolerance and mild reaction condition exhibit favorable industrialization potential. However, how to reduce catalyst dosage is a vital problem.
According to known literature reports (Chem. Rev. 2002, 102, 1359), a majority of cross-coupling reactions use a catalyst dosage of 1%-10% (U.S. Pat. No. 2,004,002,489, 2002; JP. Patent 2004,262,832, 2003; JP. Patent 2005,008,578, 2003; WO. Patent 2004,101,581, 2004; WO. Patent 2005,012,271, 2004, etc.). However, failure to reduce catalyst dosage to below 1000 ppm has a great negative effect on the product cost and the control of residue of heavy metals in final products (Chem. Rev. 2006, 106, 2651). Few literatures and patents report the use of traces of palladium catalyst to realize coupling reaction. Some excerpts are as below:
In 1991, Syntec reported that, with tri-silyl phosphate as the ligand, under the catalysis of 1000 ppm of palladium acetate, Buchwald reaction occurs between aryl bromide and amine, and the yield is high up to 91%. However, the only drawback is the need of 1 mol % of the ligand (DE. Patent 19,963,009, 1991).

In 1996, Hoechst published a patent about using traces of palladium catalyst to realize Heck reaction, in which the conversion of high yield is realized by means of the dosage of 500 ppm of catalyst for several different kinds of substrates (DE. Patent 19,647,584, 1996).

In 2001, OMG and Beller jointly developed a new-type palladium alkene coordinated catalyst to be used for Suzuki coupling reaction of chlorinated aromatic hydrocarbon and boracic acid. This catalyst is especially suitable for reaction using 2-chlorobenzonitrile as the substrate. With a dosage of 500 ppm of catalyst, the yield of this reaction can reach more than 90%. While using p-chlorofluorobenzene and p-chloroanisole as the substrate for reaction, the effect is very general (EP Patent 1,199,292, 2001).

In 2008, Hartwig et al. reported a ferrocene structure based coordination. By using it in combination with palladium acetate, a small amount of 50-2000 ppm of catalyst is enough to catalyze aryl iodide, aryl bromide and even chlorinated aromatic hydrocarbon for Buchwald-Hartwig reaction, and the yield can be high up to over 90% (J. Am. Chem. Soc. 2008, 130, 6586).

Thus it can be concluded that, the industrialized application of cross-coupling reaction highly depends on the synthesis of high-efficiency catalyst, and either an improved catalyst on the basis of the original catalyst or a catalyst with a brand new structure is of great importance.